Thin film transistor and liquid crystal display unit

ABSTRACT

A bottom-gate type thin-film transistor free from alignment shift of the gate electrode and from damage caused by injection of impurities. The crystal grains of a polycrystalline silicon thin-film are anisotropically grown to form a prescribed angle relative to the gate length direction. The angle between the gate length direction and the longitudinal direction of the grains is adjusted according to use of the liquid crystal display unit. The bottom-gate transistor includes an undercoat insulating layer containing impurities on the substrate. Impurities are diffused from the undercoat layer to the semiconductor layer by laser-annealing the amorphous silicon.

TECHNICAL FIELD

The present invention relates to a thin-film transistor (TFT) usingpolycrystalline silicon thin-film as an active layer serving as a sourcearea or a drain area, and a liquid crystal display unit (LCD) using suchTFT.

BACKGROUND ART

There is at present a demand for a liquid crystal display unit capableof displaying at a higher speed, and as one of the means for satisfyingsuch a requirement, it has been tried to change an active layer such asa gate area, a source area, or a drain area of the switching thin-filmtransistor that controls the liquid crystal layer from the amorphoussilicon thin film into the polycrystalline silicon. This is the resultobtained by paying attention to the fact that the mobility of thecarrier in polycrystalline silicone is higher than that in amorphoussilicon in principle.

In addition to formation of a polycrystalline silicon thin film having ahigh carrier mobility on an insulating substrate, it has been tried, notto externally attach a semiconductor chip having a single-crystalsilicon active layer to the driving circuit of the liquid crystaldisplay section as in the conventional art, but to simultaneously form athin-film transistor having an active layer comprising a polycrystallinesilicon thin film in the frame of the pixel section on the samesubstrate from the beginning.

We will now briefly describe with by referring to the drawings theprocess of forming a thin-film transistor by polycrystallizing thisamorphous silicon, after forming an amorphous silicon thin film on atransparent insulating substrate such as a glass substrate, and themanufacturing method of a liquid crystal display unit using a thin-filmtransistor having an active layer comprising this polycrystallinesilicon thin film, since they are related with the intent of the presentinvention although they may fall under the known conventional art.

FIG. 1 illustrates the state of changes in the cross section of aconventional thin film transistor using a polycrystalline silicon thinfilm as an active layer according to the progress of manufacture.Actually, a large number of the thin-film semiconductor for the pixelsand the driving circuits thereof are formed and arranged in many rowsand stages, i.e. the upper, lower, right, and left on the substrate inaccordance with the layout of the liquid crystal display section.However, since this is a well known fact, and moreover, it istroublesome to show this process in detail, only one thin-filmsemiconductor is shown in this FIG. 1.

In this FIG. 1, 1 represents an insulated substrate having transparencysuch as a glass, 2 represents a buffer layer for preventing an alkalimetal or the like contained in the insulated substrate 1 from diffusinginto an active layer comprising a silicon thin film and exerting anadverse Effect, 3 represents an amorphous silicon thin film; and 4represents a polycrystalline silicon thin film. 5 represents a gateinsulating film (layer) comprising, for example, SiO₂ and Si₃N₄ and 6represents a gate electrode. 7 represents a channel area and 8represents a source area. 9 represents a drain area, and 10 represents acontact hole. 11 represents a source electrode, and 12 represents drainelectrode.

A manufacturing method of a thin-film transistor using a polycrystallinesilicon thin film as an active layer of thin-film transistor will now bedescribed sequentially by referring to FIG. 1.

(a) The amorphous silicon thin film 3 shall be formed by deposition onthe insulated substrate via the buffer layer 2;

(b) Polycrystallizing treatment of silicon shall be performed byapplying a heat treatment to the amorphous silicon thin film 3 to. Moreparticularly describing, the polycrystalline silicon thin film isobtained by irradiating an excimer laser (beam) onto the amorphoussilicon thin film 3, instantaneously melting the amorphous silicon,causing crystallization in accordance with cooling, and finally applyingso-called laser annealing (or laser anneal technique). Then, unnecessaryportions of the polycrystalline silicon thin film 4 on the substrateshall be removed, and the gate insulating film 5 and the gate electrode6 shall sequentially be formed on the substrate.

In this state, impurities determining a type of conduction of thepolycrystalline silicon thin film 4 such as phosphorus (P) or boron (B)are introduced from the upper section of the substrate into thepolycrystalline silicon thin film 4 to form the source area and thedrain area of the thin-film transistor, by using the gate electrode 6 asa mask, or simultaneously using a resist together if necessary, so as toprevent impurities from entering the channel area 7. This introductionis usually accomplished by injecting P or B ions accelerated with a highvoltage. A case with P ions are illustrated in FIG. 1.

(c) The source area 8 and the drain area 9 are formed by activating theimpurities through a heat treatment carried out by irradiating anexcimer laser onto the polycrystalline silicon thin film 4 again.

(d) The source electrode 11 and the drain electrode 12 are formed byforming the contact hole 10, and by burying a metal inside it.

Next, the thin-film transistor shown in FIG. 1 is of so-called top-gatetype in which a gate insulating layer is arranged on the substrate sideof the gate electrode. As the applicable thin-film transistors for aliquid crystal unit include, apart from the top gate type, there is atype known as the bottom-gate type in which the gate insulating layer isarranged on the opposite side of the substrate against the gateelectrode.

The bottom-gate type is advantageous in that it is capable to almostperfectely prevent impurities from diffusing from the undercoat such asthe glass substrate to the channel area by means of the gate metalelectrode. In this structure, however, because impurities forming thesource area and the drain area cannot be diffused from the relativelythick substrate side, diffusion will be made from the silicon layer sideafter forming the silicon layer. As the result, it becomes difficult, oreven impossible to perform self-alignment for forming the channel area,thereby causing deterioration of the transistor characteristics, such asa increased gate capacity.

On the other hand, a favorable feature of the top-gate type is thatimpurities forming the source area and the drain area is injected, fromthe gate electrode side after forming the silicon layer, by using thegate electrode as a mask, and thereby self-alignment will be permittedfor forming the channel area. In this structure, however, since there isno gate metal under the channel area, diffusion of the impurities fromthe undercoat such as a glass substrate into the channel area during thesubsequent heat treatment cannot completely be prevented, or is at leastdifficult to completely prevent such diffusion. If the thickness of theundercoat insulating film layer on the substrate is increased to avoidthis defect, various problems such as cambering of the substrate willoccur.

A conventional bottom -ate type thin-film transistor and a manufacturingmethod of it will now be described in detail by referring to thedrawings.

FIG. 2 illustrates formation of the cross-section according to theprogress of the manufacturing process of the conventional bottom-gatetype thin-film transistor. In this FIG. 2, the numeral (latter we omit“the numeral”) 1 represents a transparent insulated substrate Comprisinga glass or the like. 5 b represents a gate insulating layer comprisingSiO₂ or the like, and 6 b represents a gate electrode. 7 b represents achannel area in the silicon semiconductor layer, and 8 b represents asource area in the silicon semiconductor layer. 9 b represents a drainarea in the silicon semiconductor layer, and 30 represents photo-resist.5 c represents an interlayer insulating layer, 11 b represents a sourceelectrode, and 12 b represents a drain electrode.

The manufacturing method thereof will be described as based on the FIG.2.

(a) The gate electrode 6 shall be formed on the transparent insulatedsubstrate 1, and then the gate insulating layer 5 b shall be formedcovering the upper section of it;

(b) The silicon semiconductor layer 4 (a necessary area only) shallselectively be formed on the gate insulating layer 5 b. In this step,when using polycrystalline silicon, which is attracting the generalattention at present, as a silicon semiconductor layer, this amorphoussilicon layer shall be polycrystallized by annealing using an excimerlaser through a laser anneal, with a laser anneal technique, forexample, after an amorphous silicon layer is formed.

Subsequently, a photo-resist 30 is formed only on the upper section ofthe silicon semiconductor layer at a position which is to constitute anupper section of the gate electrode 6 b, and then B and other impuritiesdetermining a type of conduction of silicon is injected from the uppersection of the substrate onto the silicon semiconductor layer by usingthis photo-resist 30 as a mask. As the result, the channel area 7 b inwhich impurities constituting the thin-film transistor are not existent,the source area 8 b and the drain area 9 b into which impurities areinjected will be formed.

(c) By forming an interlayer insulating layer 5 c on the entire surfaceof the substrate after removing the photo-resist, by opening the contacthole 10 at a position corresponding to the source area 8 b and the drainarea 9 b in the interlayer insulating layer 5 c, and by incorporatingthe metals such as Ti and Mo by sputtering or the like into this contacthole to form the source electrode 11 s and the drain electrode 12 b,manufacturing of the thin-film transistor will be finished.

PROBLEMS TO BE SOLVED BY THE INVENTION

However, the thin-film transistor manufactured through the steps shownin the preceding FIG. 1 cannot be made into a single silicon crystal atthe present level of art. In order to realize the liquid crystal displayon a large screen of over 12 inches to 20 inches, or a further largerscreen of 30 inches, uniformity of the thin-film transistor elements andthe functions are still insufficient, and in is turn, uniformity of thedisplay on a liquid crystal display unit and the display functions arestill also insufficient.

Next, the bottom-gate type thin-film transistor shown in FIG. 2 has thefollowing problems.

First, because a silicon semiconductor layer to serve as a source areaand a drain area is present on the upper side of the gate electrode, itis necessary to form a photo-resist as a mask at a positioncorresponding to the gate electrode which is readily formed, wheninjecting impurities into the silicon semiconductor layer, a positionalalignment at this point will also be required. However, for theextra-compact and fine thin-film transistor for the liquid crystaldisplay of higher-grade fineness in a conceivable future, it will bedifficult to conduct such a positional alignment. To describe just forinformation, a thin-film transistor at present is going to have the gatewidth of 10 μm, the length of about 6 μm, and the transistor length ofabout 20 μm, and further downsizing is expected to be realized in thefuture.

Under these circumstances, the top-gate type thin-film transistors arebecoming the mainstream.

Secondly, the technique of injection of the impurity ions acceleratedwith a high voltage is used as a means for introducing the impuritiesinto the currently polycrystallized silicon semiconductor layer,irrespective of the top-gate type or the bottom-gate type. However, thistechnique will more or less cause a damage to the crystal lattices ofthe silicon semiconductor layer. Therefore, a heat treatment is appliedfor recovering such a damage. The temperature for such a heat treatmentis, however, limited to up to about 600° C. at the maximum because ofthe heat resistance of the glass of the substrate. In its turn, it isdifficult to completely recover the damages.

Further, because of the very thin undercoat layer and semiconductorlayer, alkali metals diffuse from the glass substrate into thesemiconductor particularly during this heat treatment. Consequently,this leads to deterioration of the performance of the semiconductor.

Under these circumstances, there has been a demand for the developmentof a thin-film transistor excellent in high pixel density as well asresponse, and further having a sufficient performance in terms of theresponse and quality uniformity for the liquid crystal display unitshaving a large display screen in the conceivable future.

Whether the top gate type or the bottom gate type, there has been ademand for the development of a technique adopting an inexpensive glasssubstrate, and less susceptible to damages to the silicon layer or thelike when injecting the impurities. The demand has been particularlystrong for the bottom-gate type.

Also, in an extra-fine bottom-gate type thin-film transistor, there hasbeen a demand for the development of a technique permitting theinjection of impurities appropriately coping with the gate electrode.

In addition, there has been a demand for the achievement of aninexpensive liquid crystal display unit having a very high pixel densityand a satisfactory response with a large display area by using such athin-film transistor.

Further, by paying attention to the decrease in the melting point duringannealing and the considerable electric field movement, a newsemiconductor thin film was recently developed by adding carbon and/orgermanium located adjacent to silicon in the periodic table (5% carbonat the maximum, or up to 30% germanium a the maximum) in place of puresilicon. However, because similar problems are encountered in this caseas well, there has been a demand for solving such problems.

DISCLOSURE OF INVENTION

The present invention was developed for the purpose of solving theproblems as described above.

For this purpose, the first group of aspects of the invention provides athin-film transistor having a polycrystalline silicon thin film servingas an active area formed on an insulating substrate, in which crystalgrains of the polycrystalline silicon thin film have anisotropicallybeen grown at an array substrate end within a plane, and in a directionin parallel with, or at right angles to, the gate length direction ofthe thin-film transistor in parallel therewith, or at right anglesthereto, and the longitudinal direction thereof is at a particular angleto the gate longitudinal direction of the thin-film transistor.

When the anisotropic growth direction of the crystal grains is in thegate length direction, some of barriers are eliminated upon movement ofthe carrier, thus improving the electric field effect and mobility.

At this point, from 0.5 to 2 grains of the polycrystalline silicon thinfilm are contained per micron (1 μm) of the gate length. As the result,it is possible to achieve an electric field effect as typicallyrepresented by mobility of at least 300 cm²/Vs for all the thin-filmtransistors.

Also, uniform transistor properties are available by manufacturing thethin-film transistor so that the longitudinal direction of the grains issubstantially in parallel with the length direction of the gate of thethin-film transistor.

At this point, 5 to 20 grains are contained per micron of the gatelength. This is desirable in terms of the uniformity and a highmobility.

The longitudinal growth direction of grains forms an angle of 45° to thegate length direction of the thin-film transistor. This brings about athin-film semiconductor showing a good balance between high-speedmovement and uniform characteristics.

It is desirable that from 1 to 10 grains are contained per micron of thegate length.

From the point of view of operating efficiency, it is desirable toanneal the upper (lower) and left (right) driving circuit sections andpixel sections by a long and slender laser beam in a run of scanning,and this is adopted in the invention.

Further, the substrate or the laser beam, particularly the substrate, ismoved in a direction substantially at right angles to the longitudinaldirection, in which the intensity of the laser beam in the shorter sidedirection is stronger at least at the center portion.

Next, in a liquid crystal display unit, provision of the driving circuitof the pixel section on the same substrate leads to a lower cost as awhole. Particularly, use of the driving circuit on the data drive sidepermits achievement of a high-performance liquid crystal display unitexcellent in driving characteristics.

Further, when a shift register is provided in the driving circuitsection, a more compact liquid crystal display unit is available.Provision of a buffer circuit as required results in a further moreexcellent display unit. Use of the thin-film transistor as a liquidcrystal switch for the pixel section permits achievement of a liquidcrystal display unit giving a good contrast. Such a liquid crystaldisplay unit is proposed in the present invention.

The second group of aspects of the invention provides a bottom-gate typethin-film transistor formed on the substrate having an undercoatinsulating layer formed on the substrate, which contains impuritiesdetermining a type of the conduction of silicon, in which the impuritiescontained in the silicon layer in contact with the undercoat layer uponlaser annealing of amorphous silicon are diffused to form the sourcearea and the drain area.

Further, the presence of the undercoat insulating layer preventsimpurities contained in the transparent insulating substrate fromdiffusing into the thin-film semiconductor. As such an undercoat layer,a BSG (boron silicate glass) layer or a PSG (phosphorus silicate glass)layer is used.

In the manufacturing method of the thin-film transistor using apolycrystalline silicon semiconductor for the polycrystallization ofamorphous silicon through irradiation of an excimer laser beam, it isinstantaneously exposed to a very high temperature. A high-melting-pointmetal is therefore used as a gate electrode material.

Further, the high-melting-point metals mainly comprising Cr, Mo, Ti andthe like are used to facilitate formation of a gate side wall insulatinglayer through oxidation of the gate electrode. Moreover, since Ti or Cris passivated in oxidation (oxidizable), the thickness of the oxide filmis spontaneously controlled. This is extremely favorable in the case ofan extra-fine element.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A conceptual view of a manufacturing method of a conventionalthin-film transistor using polycrystalline silicon as an active layer oftransistor;

[FIG. 2] A conceptual view of the manufacturing method of a conventionalbottom gate type thin-film transistor;

[FIG. 3] A conceptual view of a substrate plane in which a thin-filmtransistor of the pixel section and the driving circuit section on aperipheral frame thereof of a liquid crystal display unit are integrallyformed, and the shape and the scanning direction of the laser beam onthat plane in a first embodiment of the invention;

[FIG. 4] A conceptual view of the optical system of a laser irradiatingunit, and the relationship between the beam scanning direction and theenergy density in the above-mentioned embodiment of the invention;

[FIG. 5] A conceptual plan view of the relationship between the gatelength direction and the grain growth direction of the thin-filmtransistor in the aforementioned embodiment of the invention;

[FIG. 6] A conceptual view of the cross-section of a liquid crystaldisplay unit using the thin-film transistor of the aforementionedembodiment;

[FIG. 7] A conceptual view of the substrate plane on which the thin-filmtransistor of the driving section of a liquid crystal display unit isformed, and the shape and scanning direction of the laser beam on thatplane in a second embodiment of the invention;

[FIG. 8] A conceptual plan view of the relationship between the gatelength direction of the thin-film transistor and the growth direction ofthe crystal grains in the aforementioned embodiment;

[FIG. 9] A conceptual view of the substrate plane on which the thin-filmtransistor of the driving section of a liquid crystal display unit isformed, and the shape and scanning direction of the laser beam on thatplane in a third embodiment of the invention;

[FIG. 10] A conceptual plan view of the relationship between the gatelength direction of the thin-film transistor and the growth direction ofcrystal grains in the aforementioned embodiment;

[FIG. 11] A conceptual view of the process of manufacture of atransistor and details of the laser beam in cases where theaforementioned first to the third embodiment is applied to a bottom gatetype thin-film transistor (the fourth embodiment);

[FIG. 12] A view illustrating combinations of directions of the sourceelectrode and the drain electrode of the gate driving circuit and thesource driving circuit of the driving circuit section;

[FIG. 13] A view illustrating the relationship between the beam scanningdirection and the angle formed between the substrate and the drivingcircuit;

[FIG. 14] A conceptual view of the structure of a bottom gate typethin-film transistor in the sixth embodiment of the invention; and

[FIG. 15] A conceptual view of the process of manufacture of thethin-film transistor in the sixth embodiment.

REFERENCE NUMERALS

1: Insulating substrate

2: Buffer layer (undercoat insulating film)

3: Amorphous silicon thin film (layer)

4: Polycrystalline silicon thin film

5: Gate insulating film

5 b: Gate insulating film (bottom gate)

5 c: Interlayer insulating film (bottom gate)

6: Gate electrode

6 b: Gate electrode (bottom gate)

7: Channel area

7 b: Channel area (bottom gate)

8: Source area

8 b: Source area (bottom gate)

9: Drain area

9 b: Drain area (bottom gate)

10: Contact hole

11: Source electrode

11 b: Source electrode (bottom gate)

12: Drain electrode

12 b: Drain electrode (bottom gate)

12: Pixel section

13: Gate driving circuit section

14: Source driving circuit section

15: Excimer laser beam

151: Excimer laser beam

152: Excimer laser beam

153: Excimer laser beam

16: Line showing the relative scanning direction of beam

161: Line showing the relative scanning direction of beam

162: Line showing the relative scanning direction of beam

163: Line showing the relative scanning direction of beam

20: Polysilicon thin film

21: Silicon crystal grain (long and slender polysilicon fine crystalgrain)

22: PSG layer

23: BSG layer

24: Resist pattern

30: Photo-resist

31: Laser source

32: Quonset-hut type convex lens (so called barrel roof type orlenticule type)

33: Anti-Quonset-hut type concave lens

34: One-sided Quonset-hut type lens

35: Laser beam

41: First transparent electrode

42: Transistor

43: First array substrate

44: Color filter

45: Second electrode

46: Second color filter mounting substrate

47: Alignment layer (Orientational film)

48: Spacer

49: Adhesive

410: Liquid crystal

411, 412: Polarizer plate

413: Backlight

4 p: First polysilicon layer into which impurities have been introduced

4 b: Second polysilicon layer into which impurities have been introduced

61: Gate side wall insulating layer

6 b: Gate electrode material layer

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described by means of the embodiments.

(First Embodiment)

This embodiment relates to control of the grain growth direction uponpolycrystallizing an amorphous silicon thin film through excimer laserannealing.

FIG. 3 is a plan view of a transparent insulating substrate formed in anarrangement known as one-sided mounting, comprising a thin-filmtransistor for pixel switching of the display section located at aposition slightly shifting to the right downward from substantially thecenter of the liquid crystal display unit, and thin-film transistors forthe driving circuit section of the pixel section in the upper and leftframes in the substrate.

In FIG. 3(a), 1 represents a transparent insulating substrate made ofglass or the like, and 12 represents a pixel section substantially atthe center thereof, but slightly to the right downward. 13 represents agate driving circuit (gate driver) section, and 14 represents a sourcedriving circuit (source driver) section. In the pixel section and thedriving circuit section, many thin-film transistors usingpolycrystalline silicon thin-films as active layers are arranged in aplurality of rows and stages.

The driving circuit of this embodiment contains a shift register forswitching through synchronization with a dot timing signal and displayof an image through A/D conversion.

Next, formation of the thin-film transistors of the pixel section, thegate driving circuit section and the source driving circuit section ofthis liquid crystal display unit will now be described.

First, the basic manufacturing method itself is the same as theconventional one described previously. A difference is in details ofpolycrystallization through heat treatment of the amorphous silicon thinfilm. The difference will be described in detail.

First, the excimer laser beam is a strip-shaped beam which has an energydensity uniform in the longitudinal direction, and an intensitydistribution in the short side direction. The principle for achievingthis energy density distribution of the laser beam, which is a knownart, but is directly associated with the intent of the invention, willbe shown and briefly explained.

In FIG. 3, 31 represents a laser beam source. 32 represents a convexlens forming, not so-called Quonset-hut-shaped focal point, but a focalline (one-directional). 33 represents an anti-Quonset-hut type concavelens. 34 represents one-sided Quonset-hut-shape convex lens. And, 35,35L, and 35R represents laser beams.

As shown in FIG. 3(a), a substantially linear laser beam is convertedinto a long and slender rectangular shape by the Quonset-hut type andanti-Quonset-hut type lenses along the direction of these lenses. Thedirection of this rectangle is in the same direction as that of theQuonset-hut type convex lens, i.e., it has a distribution of energydensity by causing the beam to pass through the one-sided Quonset-huttype lens which is long and slender in the longitudinal direction of thebeam. In FIG. 3(b), 35R represents this side of the long and slenderrectangular laser beam as shown in FIG. 3(a), and 35L represents anopposite direction. The amorphous silicon layer 3 to be laser-annealedis present on this one-sided convex lens 34 side slightly from the focalline. As the result, the energy density distribution of this laser beamis as shown in FIG. 3(c).

Actually, however, a slit or a reflection plate having a special shapemay be used. However, these steps are well known techniques used notonly for lasers, but also for a usual optical system, and do not coverparticularly difficult technical details. Therefore, further descriptionis omitted here for techniques for obtaining the laser beam shape andthe energy density distribution.

Next, in beam scanning, polycrystallization is carried out bysequentially heating by irradiation the amorphous silicon thin filmthrough relative movement to the substrate at a prescribed speed in adirection at right angles to the longitudinal direction whileirradiating the beam, as shown in FIG. 3(b) and FIG. 4(c). In this step,the substrate should preferably be moved, in consideration of the lasersource and the optical unit.

Next, regarding formation of the amorphous silicon layer, it is formedinto a thickness of about 100 nm by previously forming an SiO₂ layerserving as an insulating undercoat on the surface of the glasssubstrate, and forming the amorphous silicon layer by the plasma CVDmethod on this SiO₂ layer.

Then, as shown in FIG. 3(b), setting is made so that the gate lengthdirection of the thin film transistor to be formed and the movingdirection of the substrate are substantially in parallel (in FIG. 3(b),these directions are set so that the array substrate end and the beamlongitudinal direction form substantially right angles), and the beam ismoved while irradiating the beam in a direction substantially at rightangles to the laser beam longitudinal direction and in a direction inwhich the shorter side direction intensity of the laser beam isstronger.

For the reasons as described above, when moving the beam, the substrateis moved provided on the moving unit relative to the laser beam sourcefixed in the irradiating unit, hence the laser beam, therebycontinuously crystallizing the amorphous silicon thin film(step-and-repeat method).

In FIG. 3(b), 15 represents the beam shape of the excimer laser, and theline 16 indicated by arrows at the both ends represents the relativescanning directions of the beam. The both ends are shown by the arrowshere, because, depending upon the laser source output, or the beam size,it is necessary to reciprocate through the driving circuit section aplurality of times, and irradiation may be made for splashing dustdeposited onto the amorphous silicon surface with laser irradiation of alow energy density, as a preliminary stage of laser annealing.

As the result, the crystal grains of the polycrystallized silicon thinfilm (also called domain or grain), which form a circular shape in theconventional art, do not form a circle in this embodiment, but form anellipsoid long in the scanning direction.

The reason is as follows. Since the molten silicon is uniform and hasthe same temperature along the longitudinal direction, it is difficultfor heat to escape in the longitudinal direction except for the ends.Particularly when laser annealing is applied prior to patterning, moltensilicon form a long strand in the beam longitudinal direction. Heat doesnot therefore practically escape in this direction. However, in theshorter side direction which is the moving direction of the beam, theenergy distribution has a higher density toward the center. Thetemperature therefore becomes lower in a direction counter to theadvancing direction of the beam. Upon solidification of silicon,therefore, silicon is considered to be cooled from the beamanti-advancing direction end toward the running direction end.

Next, irradiating conditions will be described.

For example, when irradiation is conducted by using KrF (this may beXeCl) excimer laser under conditions including a thickness of theprecursor amorphous silicon (a-Si) of 100 nm, a substrate temperature of500° C., and a laser irradiating energy of 330 mJ/cm². Opticalmicroscopic confirmation after a set scanning movement at a pitch of 1micron/shot suggests that there is available a thin film formed bysilicon fine crystal grains having a longitudinal direction in thescanning direction, a longitudinal grain size within a range of from 3to 5 microns and a shorter side grain size within a range of from 0.5 to2 microns.

A higher substrate temperature is preferable, but when using glass, thetemperature of the substrate itself should be within a range of from300° C. to 600° C. considering the heat resistance of the material, andthe most desirable polycrystalline silicon can be obtained underconditions including a film thickness within a range of from 30 to 200nm, a laser irradiating energy within a range of 280 to 420 mJ/cm², anda laser irradiating power distribution within a range of about 3 to 10mW/cm² per 10 microns.

Then, using the polycrystalline silicon thin film thus formed, athin-film transistor can be formed through the same steps as those shownin FIG. 1. As a result, a polycrystaline silicon thin-film transistor,in which grains have anisotropically grown as shown in FIG. 5, isobtained.

In FIG. 5, 11 represents a source electrode, 12 represents a drainelectrode, 6 represents a gate electrode, and 20 represents acrystallized polysilicon thin film. And, the small ellipsoid 21 in FIG.5 represents a silicon crystal grain anisotropically grown, and thearrows at both ends shown by thick black lines indicates a gate lengthdirection.

The thin-film transistor of this embodiment, comprising silicon crystalgrains having grown long in the gate length direction, permitsminimization of the grain boundaries of the grains present in thechannel area of the thin-film transistor. Barriers to the movement ofthe carrier when operating the thin-film transistor are thereforereduced (microscopically, approaching a single crystal), and there isavailable a thin-film transistor group having a high electric fieldeffect mobility (specifically, about 480 cm²/Vs). The resultant TFT isexcellent for a liquid crystal display unit of a television set or ananimation display unit.

(Case of Application of the First Embodiment)

This case of application relates to a liquid crystal display unit usingthe thin-film transistor of the first embodiment of the invention.

FIG. 6 shows the liquid crystal display unit of this case ofapplication. In the manufacturing method of this liquid crystal displayunit, a liquid crystal layer is held between a first array substrate 43having the first electrode 41 placed in a matrix shape and thetransistor group 42 prepared in this embodiment, for driving thiselectrode group, and the second color filter unit substrate 46 havingthe R, G, and B color filters 44 placed opposite to the first electrodeand the second electrode 45. In this step, the alignment films 47 and 47having an alignment effect on the liquid crystal are previously formeddirectly on the inner surfaces of the both substrates (the sides of theliquid crystal layers) or on the surface of a thin-film formed for anyother purpose.

Also, the first and second color filter substrates 43 and 46 arepreviously aligned so that the electrodes face each other, and securedwith a gap of about 5 microns by means of the spacer 48 and the adhesive49.

Then, a liquid crystal layer 410 is formed by injecting by the vacuummethod TN liquid crystal (ZL14792: made by Merk Co.) between the firstand second substrates 43 and 46, and then, polarizer plates 411 and 412are combined to complete a liquid crystal display element.

An image is displayed in the arrow A direction by driving the individualtransistors by use of the video signals while irradiating the backlight413 shown by an arrow over the entire surface in this liquid crystaldisplay unit.

Meantime, as to the performance of this liquid crystal display unit,comparison of delays is signal processing even by enlarging the liquidcrystal screen as a whole from 3.8 type to 13 type, suggests that it ispossible to minimize the delay, and the possibility of performing ahigh-speed display has been confirmed.

(Second Embodiment)

This embodiment as well relates to control of the growing direction ofthe silicon grains upon polycrystallizing silicon through excimer laserannealing. However, the growth direction is different when compared withthat of the above-mentioned first embodiment.

The liquid crystal display unit of this embodiment as shown in FIG. 7(a)has the same substrate as that shown in FIG. 3(a), and consequently thesame reference numerals are assigned to the corresponding componentparts.

Furthermore, the thin-film transistors of the gate driving circuitsection 13 and the source driving circuit section 14 are both thethin-film transistors for driver circuit, and contain shift registers asin the above embodiment.

Further, the manufacturing method of the polycrystalline thin-filmsemiconductor of this embodiment comprises the steps of previouslyforming an SiO₂ layer on the surface of a glass substrate, forming anamorphous silicon layer into a thickness of about 100 nm by the plasmaCVD method via the SiO₂ layer, rectifying an excimer laser beam into astrip shape having an energy uniform in the longer side direction and anintensity distribution in the shorter side direction, and moving thelayer in a direction substantially at right angles to the longer sidedirection thereof while irradiating the beam to irradiate and heat theamorphous silicon thin film, thereby conducting polycrystallization: thebasic steps are the same as in the conventional art and in the firstembodiment described above.

Next, this is followed by the steps of setting the directions so thatthe gate length direction of the TFT to be formed is substantially inparallel with the moving direction of the substrate, and moving thesubstrate in a direction substantially at right angles to the longerside direction of the laser beam which is a direction in which theintensity of the laser beam in the shorter side direction is stronger,irradiating the excimer laser beam each time the substrate is movedrelative to the excimer laser, thereby continuously crystallizing theamorphous silicon thin film, as in the first embodiment mentioned above.

However, growth anisotropy of the grains upon polycrystallizing theformed amorphous silicon thin film by heat treatment is different. Thedifference will therefore be described here.

The scanning direction of the excimer laser beam in this embodiment isshown in FIG. 7(b). In FIG. 7(b) as well, 15 represents the shape of theexcimer laser beam, and the line 16 having arrows at the both endsrepresents the scanning direction of the excimer laser relative to thesubstrate or the pixel, or bearings. As is clear from FIG. 7, first ofall in this embodiment, scanning is conducted in a directionperpendicular to that in the above-mentioned embodiment. Secondly, thesource driving circuit section 14 and the pixel section aresimultaneously laser-annealed with a laser beam in the same scanningdirection.

Meantime, because the heating treatment is applied by using an excimerlaser having an intensity distribution in the shorter side direction,the crystal grain of the polycrystallized silicon thin film does nottake a circular shape, but has a longer side direction and a shorterside direction, and takes the shape of an ellipsoid long and slender inthe scanning direction. Unlike in the above-mentioned first embodiment,however, the growth direction of silicon grains is at right angles tothe gate length direction. This is illustrated in FIG. 8.

In FIG. 8, 6 represents a gate electrode, 11 represents a sourceelectrode, and 12 represents a drain electrode. And, 20 represents acrystallized polysilicon thin film, and the small ellipsoid is ananisotropically grown silicon crystal grain 20.

Next, also in this embodiment, irradiation conditions include, forexample, KrF (this may be XeCl) excimer laser; a thickness of theprecursor amorphous silicon (a-Si) of 100 nm, a substrate temperature of500° C., a laser irradiation energy of 330 mJ/cm², and a scanningmovement pitch of 1 micron/shot. After irradiation, confirmation is madethrough an optical microscope, and there is available a thin filmcomprising silicon fine grains having a longer side direction in thescanning direction and having a grain size within a range of from 3 to 5microns in the longer side direction, and a grain size within a range offrom 0.5 to 2 microns in the shorter side direction.

The optimum conditions at this point include, as in the firstembodiment, the highest possible temperature, or a substrate temperaturewithin a range of from 300 to 600° C. when glass is used, a filmthickness within a range of from 30 to 200 nm, a laser irradiationenergy within a range of from 280 to 420 mJ/cm², and a laser beamirradiation power in the shorter side direction within a range of fromabout 3 to 10 mW/cm² per 10 microns.

Then, a polycrystalline silicon thin-film transistor as shown in FIG. 1was manufactured, as in the first embodiment described above by usingthe thus formed polycrystalline silicon thin film.

In this thin-film transistor, as shown in FIG. 8, the gate lengthdirection of the thin-film transistor is substantially at right anglesto the longitudinal direction of the polycrystalline silicon crystalgrains grown and formed in a long and slender shape. As the result, itis possible to minimize the grain boundaries present in the channel areaof the thin-film transistor.

It is therefore possible to provide an LCD excellent in displayuniformity, in which barriers for carrier movement are made uniform uponoperating the thin-film transistor. This results in a TFT excellent fora liquid crystal display unit exclusively used for displaying in a wordprocessor, displaying a static image, a guiding sign in an electric caror a similar image.

Then, a liquid crystal display unit for a word processor using thethin-film transistor of this embodiment was manufactured. The resultantunit was really excellent. However, because this liquid crystal displayunit has the same configuration as that shown in FIG. 6, illustrationwith drawings is subsequently omitted.

(Third Embodiment)

This embodiment as well relates to control of growth orientation of thegrains upon polycrystallizing through excimer laser annealing. Ascompared with the two preceding embodiments, however, a difference isthat the direction thereof forms an angle toward the gate lengthdirection by 45°. The thin-film transistor of this embodiment will bedescribed with reference to the drawings.

The substrate for the liquid crystal display unit of this embodiment, asshown in FIG. 9(a), is the same as those in the two precedingembodiments as shown in FIGS. 3(a) and 7(a).

Further, in this embodiment as well, the basic manufacturing steps andthe features, or more specifically, rectifying the strip-shaped laserbeam having uniform energy in the longer side direction and an intensitydistribution in the shorter side direction, subjecting the amorphoussilicon thin-film to irradiation and heating while irradiating thislaser beam and while moving it in a direction substantially at rightangles to the longer side direction, thereby performingpolycrystallization while keeping anisotropy of growth of the liquidcrystal grains, irradiating conditions, and crystal shape and size arethe same as in the two preceding embodiments. A difference lies,however, in the grain orientation upon polycrystallizing the previouslyformed amorphous silicon thin film.

More particularly, as shown in FIG. 9(b), setting is made so that theside of the array substrate and the beam longer side direction form anangle of 45°, i.e., the gate longer side direction of the thin-filmtransistor to be formed and the substrate moving direction formsubstantially 45°, and the substrate or the laser beam is moved whileirradiating the laser beam in a direction substantially at right anglesto the longitudinal direction of the laser beam, in which intensity oflaser beam in the shorter side direction becomes the strongest.

Also in FIG. 9(b), 15 represents the shape of the excimer laser beam,and the line 16 having arrows at both ends represents the scanningdirection of the beam relative to the substrate.

As the result, in this embodiment as well, the crystal grainpolycrystallized forms an elliptical shape long in the scanningdirection. The longitudinal direction of the grain itself inclines by45° toward the gate length direction as shown in FIG. 10.

Then, a TFT will be formed by using the polycrystalized silicon thinfilm formed as in the same manner as described above in the twopreceding embodiments.

Since, in the thin-film transistor of this embodiment, the gate lengthdirection of the thin-film transistor forms an angle of approximately45° to the longitudinal direction of the crystal grains ofpolycrystalline silicon grown and formed into a thin and slender shape,a high electric field effect mobility is available while reducing thegrain boundary dispersion of the grains present in the channel area ofthe thin-film transistor.

Next, a liquid crystal display unit for word processor using thethin-film transistor of this embodiment was manufactured, and a veryexcellent one was obtained. The configuration of this liquid crystaldisplay unit, being the same as that shown in FIG. 6, is not expresslyillustrated in a drawing here.

(Fourth Embodiment)

The unit of this embodiment is the same as those presented in the threepreceding embodiments in terms of the anisotropic growth of crystalgrains. The only difference is that the transistor of this embodiment isof the bottom gate type.

The manufacturing method of this embodiment will be described byreferring to FIG. 11 as the following.

(a) Forming a gate electrode 6 b at a prescribed position on a glasssubstrate 1 having a transparent undercoat insulating film 2 formedthereon;

(b) Forming a gate insulating film 5 b comprising SiO₂ on the entiresurface of the substrate;

(c) forming an amorphous silicon layer 3 on the entire surface of thesubstrate;

(d) Converting the amorphous silicon layer into a polycrystallinesilicon layer 4 by irradiating an excimer laser; and

(e) Patterning the polycrystalline silicon layer 4, and forming a thickresist pattern 24 on the upper portion of the gate electrode 5 b; inthis state, injecting ions of impurities such as P and B from the upperportion of the substrate.

Subsequently, an interlayer insulating film is formed, and a contacthole is formed at a prescribed position of this interlayer insulatingfilm. Then, this contact hole is filled with a metal to form a bottomgate type thin-film transistor as shown in FIG. 2(c).

Meantime, as to irradiation of the excimer laser as shown in FIG. 11(d),the beam 15 thereof has the same long and slender rectangular shape asin the embodiments described above as shown in FIG. 11(d) to the right,and the scanning direction 16 is at right angles to the longitudinaldirection. However, the distribution of the energy density is differentfrom that shown in FIG. 4(c): the curve is high at the center portionand low at both ends in the scanning direction and in thecounter-scanning direction. Such an energy distribution is adoptedbecause, while the beam must move sequentially on the entire surface ofthe substrate to scan the entire surface of the substrate, moving thebeam in a direction perpendicular to the longitudinal direction causesthe density spontaneously becomes higher at the center portion in theshorter side direction. (Note that in an actual manufacture, thesubstrate is moved relative to the fixed beam as described above.)

Unlike the top-gate type, the gate electrode section is necessary tomake irradiation so as to avoid production of shade as a result thereofin the bottom gate type. In this embodiment as well, crystal grainsbecome valid along the scanning direction of the beam. As the result,anisotropic growth of crystal grains in the same manner as in thepreceding embodiments is accomplished, and hence a bottom-gate typethin-film semiconductor having the same features is available, dependingupon whether the scanning direction is in the channel direction on thesubstrate, in a direction perpendicular thereto, or in a direction at anangle of 45°.

(Fifth Embodiment)

This embodiment relates to a combination of the gate length directionsof the gate driving circuit section and the source driving circuitsection, and laser annealing to cope therewith.

This embodiment will be described by referring to FIG. 12 as thefollowing.

As shown in FIGS. 12(a) to 12(d), there are four combinations of thegate length directions which connect the source electrode and the drainelectrode. In FIG. 12, 9 represents a source electrode of TFT; D, adrain electrode; G, a gate electrode; and the line having arrow at bothends between S and D represents the gate length direction. Anappropriate combination is selected according to the use of the liquidcrystal display unit.

Next, specifically taking the combination shown in FIG. 12(a) as anexample, the beam scanning direction upon laser annealing may be any ofthe three combinations, as represented by arrows at both ends of 161,162, and 163 shown in FIG. 13: the beams 161, 162, and 163 led inparallel with the individual gate driving circuit sections 13; inparallel with the source driving circuit sections 14; and movement inthe longitudinal direction at an angle of 45° to the both circuitsections. As the result, the amorphous silicon thin film on the surfaceof the substrate is polycrystallized by a single run of laser beamscanning. Since a smaller amount of energy (E) per unit area suffices inthis case for a thin-film transistor as compared with a thick-filmtransistor, it is needless to mention that no problem is encountered inthe capacity of the laser source even with a wide substrate to beannealed.

(Sixth Embodiment)

This embodiment relates to a bottom-gate type thin-film transistor.

FIG. 14 illustrates the sectional structure of the thin-film transistorof this embodiment. In FIG. 14, 1 represents a glass transparentinsulating substrate. On the substrate, the PSG layer 22 serving as anundercoat layer containing conductive impurities determining aconduction type of silicon, and the BSG layer 23 serving as an undercoatlayer similarly containing another conductive impurities are selectively(at necessary portions in conformity to the layout of the elements)formed. The gate electrode 6 b comprising a high-melting-point metalsuch as chromium, the first polysilicon layer 4 p into which impuritiesare introduced by diffusion of P (phosphorus) from the PSG layer 22, andthe second polysilicon layer 4 b into which impurities are introduced bydiffusion of B (boron) from the BSG layer 23, for example, are formed onthe PSG layer 22 and the BSG layer 23. The first polysilicon layer 4 pand the second polysilicon layer 4 b constitute a source area 8 b and adrain area 9 b, respectively, of the thin-film transistor.

Further, the gate side wall insulating layers 61 for electricallyinsulating between the gate electrode and the first polysilicon layerinto which impurities have been introduced, and between the gateelectrode and the seconds plysilicon layer into which impurities havebeen introduced, are formed on the side of the gate electrode. The gateside wall insulating layer should preferably be an insulating layerobtained by oxidizing the gate electrode from the point of view ofmanufacture.

An insulating film 5 b serving as a gate insulating layer is formed onthe gate electrode 6 b. A polysilicon layer into which impurities arenot introduced is further formed thereon to serve as a channel area 7 bof the thin-film transistor, so as to cover the gate insulating film 5 band the gate side wall insulating layer 61, and so as to come intocontact with the first polysilcon layer 4 p into which P has beenintroduced and the second polysilicon layer 4 b into which B has beenintroduced.

In this element, an interlayer insulating layer 5 c is, further, formedthereon, and the contact hole 10 is formed at a necessary position ofthis insulating layer. The source electrode 11 b and the drain electrode12 b are formed by incorporating a metal into this contact hole 10.

Next, as is clear from the above description, this thin-film transistoruses heat diffusion of P and B from the undercoat layer containing theimpurities determining the type of conduction such as the PSG layer andthe BSG layer for forming the source area and the drain area. As theresult, unlike the conventional one, the manufacturing method of thisembodiment does not have a step of injecting impurity ions throughacceleration under a high voltage into the polysilicon layers. Damagesdo not occur to the silicon semiconductor layer, caused by collision ofions accelerated under the high voltage. Because a photo-resist is notused as a mask for injection of impurities, no shift of alignmentoccurs, thus permitting forming the source area and the drain area ataccurate positions.

The PSG layer and the BSG layer serving as undercoat film prevent alkalimetals existing in the transparent insulating substrate which is a glasssubstrate from diffusing into the silicon semiconductor layer.Furthermore, as compared with the undercoat layer such as a siliconnitride film conventionally used for the purpose of preventingdiffusion, the diffusion preventing effect of an alkali metal is higher.As the result, for example, during heat treatment in the latter stage ofmanufacture of the thin-film transistor, or more specifically, in theactivation treatment after impurity ion injection, it is possible toprevent diffusion of alkali metals or the like exerting an adverseeffect on the properties from diffusing from the glass substrate withoutfail.

It is needless to mention that the PSG layer or the BSG layer may beformed on the undercoat layer such as a silicon nitride film.

Next, the manufacturing method of the thin-film transistor of thisembodiment will be described by referring to FIG. 15. FIG. 15illustrates the progress of manufacture of the thin-film transistoralong with development of the manufacturing steps.

(a) Forming the PSG film 22 containing about 3.5% P which is an N-typeimpurity as an undercoat insulating film in an area for forming aleft-side N channel transistor; and forming the BSG film 23 containingabout 35% B which is a P-type impurity as an undercoat insulating filmin an area for forming a right-side P channel transistor.

Furthermore, the gate electrode material layer 6 c comprising a Cr(chromium) film will be formed on these PSG film and the BSG film.

Subsequently, forming the gate insulating layer 5 b such as SiO₂ on theentire surface, and for the purpose of forming a gate electrode pattern,the resist layer 24 will be formed only on the corresponding portion.That is to say, a resist pattern will be formed.

(b) Applying dry etching with the thus formed resist pattern 24 as amask, the gate electrode 6 b will be formed. In this case, a gateinsulating layer will simultaneously be patterned.

(c) Removing the resist pattern used upon etching, the gate electrodeside wall insulating layer 61 will be formed by oxidizing the gateelectrode 6 b by heating. This gate electrode side wall insulating layer61 electrically insulates between a silicon, a silicon semiconductorlayer formed thereafter and the gate electrode. In this embodiment, notonly simple heating makes it easier to cause oxidation and form the gateelectrode side wall insulating layer, but also passivation of the metaloxides causes stoppage of the oxidation when a certain oxide filmpressure is reached, thus eliminating the risk of excessive oxidationeven with an extra-fine particles. For these properties and heatresistance, Cr is used as a material.

(d) a semiconductor layer serving as a channel area, a source area or adrain area of the thin-film transistor will be formed. Specifically, anamorphous silicon film 3 is deposited on the entire surface of thesubstrate 1.

(e) Irradiating an excimer laser beam shown by an arrow onto theamorphous silicon layer 3 to accomplish polycrystallization thereof; theexcimer laser irradiation area is thereby heated to a very hightemperature, though instantaneously, but it is needless to mention thatCr having a high melting point used as a gate electrode material nevermelts.

Meatime, simultaneously with polycrystallization by means of the excimerlaser beam, the source area and the drain area are subjected to anactivation treatment through diffusion of impurities. More specifically,impurities P and B diffuse to the polycrystalline silicon layer from thePSG film and the BSG film at a high temperature resulting fromirradiation of the excimer laser, thereby forming the source area andthe drain area.

In this embodiment, as is clear from the above description, irradiationof the excimer laser permits simultaneous accomplishment ofpolycrystallization of the amorphous silicon layer and introduction ofimpurities into the source area and the drain area which are transistorelements. This eliminates the necessity of a step of accelerating andinjecting impurities under a high voltage, and a step of bonding with adangling bond (heat treatment), apart from polycrystallization. Since amask used in the conventional art is not used at this point, the processis free from a shift between the mask and the gate electrode, thuspermitting accurate formation of the source are and the drain area. Therisk of damages to the semiconductor caused by collision of impuritiesor hydrogen ions accelerated under the high voltage during ion doping isalso eliminated.

In FIG. 15(e), the thickness of the polycrystalline silicon layerpresent on the side of the gate electrode is larger than that of thepolycrystalline silicon layers of the other areas. By properlycontrolling the heat treatment conditions or the like, therefore, theconcentration of impurities P and B becomes lower toward the upper sideof the substrate, thereby permitting automatic formation of an LDDstructure.

(f) The interlayer insulating layer 5 c comprising, for example, SiO₂,will be formed on the entire surface of the transparent insulatingsubstrate having the gate electrode, the source area, and the drain areaformed thereon.

(g) Forming the contact holes 10 at positions corresponding to thesource area 8 b and the drain area 9 b of the interlayer insulatinglayer 5 c, and incorporating a metal into the contact holes, the sourceelectrode 11 b and the drain electrode 12 b will be formed, and therebythe thin-film transistor will be completed.

In this embodiment, the invention has been described as to a case wherethe silicon semiconductor layer composing the active layer of thethin-film transistor is a polycrystalline silicon layer. This layer isnot, however, limited to a polycrystalline silicon layer, but maycomprise amorphous silicon, silicon-germanium, orsilicon-germanium-carbon.

Further, the gate insulating layer on the top of the gate electrode maybe formed by oxidizing a gate electrode material such as Cr.

Similarly, the above description is based on a case where the N-channeltransistor and the P-channel transistor are formed simultaneously on thesubstrate. However, only one of them may be formed.

The present invention has been described as above by means of a fewembodiments, but it is needless to mention that the invention is notlimited by these embodiments. For example, any of the following featuresmay be adopted:

1) A crystallization accelerating agent such as trace Ti, Ni or Pd isadded for reducing the temperature of polycrystallization of silicon(causing solid-phase growth);

2) Annealing is conducted by means other than laser such as electronbeam for heating to polycrystallize silicon or to diffuse impurities;

Particularly, in the second group of aspects of the invention, heatingis performed by means of an electric heater, or through simultaneous useof an electric heater;

3) The driving circuit section of the liquid crystal display unit is notlimited to a one-sided mounting, but may be formed on each of all theframes including the upper, lower, right, and left frames;

4) Combining the fifth embodiment and the sixth embodiment; that is tosay, the impurities are diffused from the undercoat layer, and at thesame time with this, the longitudinal growth direction of thesemiconductor grains forms a prescribed angle to the gate lengthdirection;

5) The liquid crystal display unit is of the other type including thereflection type.

INDUSTRIAL APPLICABILITY

According to the present invention, as described above, it is possibleto manufacture a high-performance TFT array having features such as ahigh electric field effect mobility of each TFT and uniform propertiesof TFTs by forming a polycrystalline semiconductor thin film obtainedthrough excimer laser annealing of a silicon thin film to comprise longand slender fine grains, and providing a prescribed angle between thegrain longitudinal direction and the gate length direction of themanufactured TFT. As the result, when using a liquid crystal displayunit having a large screen, it is possible to obtain a TFT sufficientlysatisfying property requirements taking account of the use of the unit.

Further, in a bottom-gate type semiconductor, since the impurities aresoaked from the undercoat layer by heat diffusing, it is not necessaryto provide an injector for injecting impurity ions under a high voltageand a heat treatment step to be required along with this, and inaddition, damages to the silicon semiconductor layer caused by collisionof the ions accelerated under a high voltage can be eliminated. It willalso become possible to form the source area and the drain areaaccurately at the positions corresponding to the gate electrodes.

Further, an inexpensive glass substrate is used as a transparentinsulating substrate. By using a PSG layer and a BSG layer as undercoatlayers containing impurities, it is possible to prevent alkali metalsand the like affecting properties of the thin-film transistor fromdiffusing from the glass substrate to the silicon semiconductor layerwithout fail.

Therefore, it is possible to provide a TFT excellent in its reliabilityand properties.

What is claimed is:
 1. A thin-film transistor (TFT) comprising, as anactive area, a polycrystalline semiconductor thin film formed bycrystallizing an amorphous semiconductor material deposited on asubstrate with laser annealing technique which conducts scanning in adirection at right angle to the longitudinal direction with a beamhaving a strip or rectangular shape, and having an energy densitydistribution in a shorter side direction, and uniform in thelongitudinal direction; wherein: the crystal grains of saidsemiconductor thin film have been anisotropically grown into a longellipsoid having a longer diameter of from 3 to 5 microns and a shorterdiameter of from 0.5 to 2 microns, of which the longitudinal growthdirection forms a certain angle relative to the gate length directionconnecting a source electrode and a drain electrode of each TFT in thesubstrate plane.
 2. A thin-film transistor according to the claim 1,wherein the longitudinal direction of the crystal grains of saidpolycrystalline silicon thin film is in parallel with the gate lengthdirection of the thin-film transistor.
 3. A thin-film transistoraccording to the claim 2, wherein said polycrystalline semiconductorthin film contains from 0.5 to 2 crystal grains per micron gate length.4. A thin-film transistor according to the claim 1, wherein, in saidpolycrystalline semiconductor thin film, the longitudinal growthdirection length of the crystal grain is longer than the gate length. 5.A thin-film transistor according to claim 1, wherein saidpolycrystalline semiconductor thin film comprises silicon,silicon-germanium, or silicon-germanium-carbon.
 6. A thin-filmtransistor according to the claim 4, wherein said polycrystallinesemiconductor thin film comprises silicon, silicon-germanium, orsilicon-germanium-carbon.
 7. A thin-film transistor according to theclaim 1, wherein the longitudinal growth direction of the crystal grainsof said polycrystalline silicon thin film forms right angle to the gatelongitudinal direction of the thin-film transistor.
 8. A thin-filmtransistor according to the claim 7, wherein said polycrystallinesemiconductor thin film contains from 5 to 20 crystal grains per micronof the gate length.
 9. A thin-film transistor according to the claim 7,wherein said polycrystalline semiconductor thin film comprises silicon,silicon-germanium, or silicon-germanium-carbon.
 10. A thin-filmtransistor according to the claim 1, wherein the longitudinal directionof the crystal grains of said polycrystalline silicon thin film isinclined by 45° in the gate length direction of the thin-filmtransistor.
 11. A thin-film transistor according to the claim 10,wherein said polycrystalline semiconductor thin film contains from 1 to10 crystal grains per micron of the gate length.
 12. A thin-filmtransistor according to the claim 10, wherein said polycrystallinesemiconductor thin film comprises silicon, silicon-germanium, orsilicon-germanium-carbon.
 13. A thin-film transistor (TFT) which is athin-film transistor of at least any one of the pixel section and theperipheral driving circuit section of a liquid crystal display unithaving, as an active area, a polycrystalline semiconductor thin filmformed by crystallizing an amorphous semiconductor material deposited ona substrate by a laser annealing technique which conducts scanning in adirection at right angle to the longitudinal direction with a beamhaving a strip or rectangular shape, and having an energy densitydistribution in a shorter side direction, and uniform in thelongitudinal direction, wherein: the crystal grains of saidsemiconductor thin film have been anisotropically grown into a longellipsoid having a longer diameter of from 3 to 5 microns and a shorterdiameter of from 0.5 to 2 microns, of which the longitudinal growthdirection forms a certain angle relative to the gate length directionconnecting a source electrode and a drain electrode of each TFT in thesubstrate plane.
 14. A thin-film transistor according to the claim 13,wherein the longitudinal direction of the crystal grains of saidpolycrystalline silicon thin film is in parallel with the gate lengthdirection of the thin-film transistor.
 15. A thin-film transistoraccording to the claim 14, wherein said polycrystalline semiconductorthin film contains from 0.5 to 2 crystal grains per micron of the gatelength.
 16. A thin-film transistor according to the claim 13, wherein,in said polycrystalline semiconductor thin film, the longitudinal growthdirection length of the crystal grain is longer than the gate length.17. A thin-film transistor according to any claim 13, wherein saidpolycrystalline semiconductor thin film comprises silicon,silicon-germanium, or silicon-germanium-carbon.
 18. A thin-filmtransistor according to the claim 16, wherein said polycrystallinesemiconductor thin film comprises silicon, silicon-germanium, orsilicon-germanium-carbon.
 19. A thin-film transistor according to theclaim 13, wherein the longitudinal growth direction of the crystalgrains of said polycrystalline silicon thin film forms right angle tothe gate longitudinal direction of the thin-film transistor.
 20. Athin-film transistor according to the claim 19, wherein saidpolycrystalline semiconductor thin film contains from 5 to 20 crystalgrains per micron of the gate length.
 21. A thin-film transistoraccording to the claim 19, wherein said polycrystalline semiconductorthin film comprises silicon, silicon-germanium, orsilicon-germanium-carbon.
 22. A thin-film transistor according to theclaim 13, wherein the longitudinal direction of the crystal grains ofsaid polycrystalline silicon thin film is inclined by 45° in the gatelength direction of the thin-film transistor.
 23. A thin-film transistoraccording to the claim 22, wherein said polycrystalline semiconductorthin film contains from 1 to 10 crystal grains per micron of the gatelength.
 24. A thin film transistor according to the claim 22, whereinsaid polycrystalline semiconductor thin film comprises silicon,silicon-germanium, or silicon-germanium-carbon.
 25. A thin-filmtransistor according to the claim 14 or 15, wherein said liquid crystaldisplay unit is a liquid crystal display unit for a television set orfor an animation display unit.
 26. A thin-film transistor according tothe claim 16, wherein said liquid crystal display unit is a liquidcrystal display unit for a television set or for an animation displayunit.
 27. A thin-film transistor according to the claim 17, wherein saidliquid crystal display unit is a liquid crystal display unit for atelevision set or for an animation display unit.
 28. A thin-filmtransistor according to the claim 18, wherein said liquid crystaldisplay unit is a liquid crystal display unit for a television set orfor an animation display unit.
 29. A thin-film transistor according tothe claim 19 or 20, wherein said liquid crystal display unit is a liquidcrystal display unit for a word processor, or for the display of astatic image, or for the guidance in an electric train, or the like. 30.A thin-film transistor according to the claim 21, wherein said liquidcrystal display unit is a liquid crystal display unit for a wordprocessor, or for the display of a static image, or for the guidance inan electric car, or the like.
 31. A thin-film transistor according tothe claim 22 or 23, wherein said liquid crystal display unit is a liquidcrystal display unit for a word processor, or for the display for astatic image, or for the guidance in an electric car, or the like.
 32. Athin-film transistor according to the claim 24, wherein said liquidcrystal display unit is a liquid crystal display unit for a wordprocessor.